1. Field of the Invention
This invention relates generally to the treatment of heart defects by the administration of electrical therapy. More particularly, this invention relates to an energy-delivery apparatus for imparting the electrical therapy to the heart.
2. Description of the Related Art
Technology is available for correcting excessively slow heart rates (bradycardia) using implantable devices, commonly referred to as pacemakers, which deliver microjoule electrical pulses to a slowly beating heart in order to speed the heart rate up to an acceptable level. Also, it is well known to deliver high energy shocks (e.g., 180 to 360 joules) via external paddles applied to the chest wall in order to correct excessively fast heart rates, and prevent the possible fatal outcome of ventricular fibrillation or certain ventricular tachycardias. Bradycardia, ventricular fibrillation, and ventricular tachycardia are all electrical malfunctions (arrhythmias) of the heart. Each may lead to death within minutes unless corrected by the appropriate electrical stimulation.
One of the most deadly forms of heart arrythmias is ventricular fibrillation, which occurs when the normal, regular electrical impulses are replaced by irregular and rapid impulses, causing the heart muscle to stop normal contractions and to begin to quiver. Normal blood flow ceases, and organ damage or death may result in minutes if normal heart contractions are not restored. Although frequently not noticeable to the victim, ventricular fibrillation is often preceded by ventricular tachycardia, which is a regular but fast rhythm of the heart. Because the victim has no noticeable warning of the impending fibrillation, death often occurs before the necessary medical assistance can arrive.
Because time delays in applying the corrective electrical treatment may result in death, implantable pacemakers and defibrillators have significantly improved the ability to treat these otherwise life threatening conditions. Being implanted within the patient, the device continuously monitors the patient's heart for treatable arrhythmias and when such is detected, the device applies corrective electrical pulses directly to the heart.
Normal heart function often can be restored to a person suffering ventricular fibrillation or ventricular tachycardia by a procedure known as cardioversion, the synchronized application of electric therapy to the heart muscle. Pacemakers and defibrillators that apply corrective electrical pulses externally to the patient's chest wall also are used to correct such life-threatening arrhythmias but suffer from a drawback insofar as it may not be possible to apply the device in time during an acute arrhythmic emergency to save the patient's life. Such treatment is needed within a few minutes to be effective.
Consequently, when a patient is deemed at high risk of death from such arrhythmias, electrical devices often are implanted so as to be readily available when treatment is needed. Alternatively, such patients are kept in a hospital where corrective electrical therapy is generally close at hand. Long term hospitalization, however, is frequently impractical due to its high cost, or due to the need for patients to engage in normal daily activities.
There also are many patients susceptible to heart arrhythmias who are at temporary risk of sudden death. For example, patients who have suffered a myocardial infarction are at substantial risk of tachyarrhythmias for several weeks thereafter. Such patients generally are hospitalized but could be discharged earlier if there were a practical means to protect them from life threatening arrhythmias. Additionally, patients awaiting implantation of an automatic defibrillator may require an external defibrillator to be close at hand, in case they experience a life-threatening tachyarrhythmia. Furthermore, some patients who may benefit from an implantable defibrillator may face an inordinate risk from the surgery required for implanting such a device.
It is evident from the above that there is a need for providing an effective means whereby susceptible patients can receive timely defibrillation or cardioversion without having to undergo an implant procedure and without having to remain hospitalized. Such a device should be capable of determining the energy as it is being delivered to the patient in order to monitor effective treatment, as well as determine that the device is in proper operating condition for delivering these therapeutic energy pulses to the patient.